Switching an LLM pipeline to structured outputs killed a months-long silent bug
ClickOk5811 · reddit · 2026-09-07
A Reddit developer described a silent bug that had cost them for months: one LLM call returned a classification as free text, which the next call consumed and acted on. In production, tiny formatting variance (capitalization, extra words) caused intermittent downstream match failures that quietly fell back to generic behavior—while both calls looked correct in isolation and all tests passed.
The fix: switching the classifier to structured outputs with an enum made the variance structurally impossible—the model must return one of the defined values. Same task, same model, and the bug simply can't exist anymore. The author's takeaway: any output consumed by another prompt or code, not read by a human, should default to structured outputs, reserving free text only for the final human-facing step.
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